The present invention relates to a nuclear magnetic resonance spectrometer, in particular a nuclear spin tomograph serving to investigate biological samples, comprising a magnetic coil for generating a substantially homogeneous magnetic field in a volume under examination, and a set of shim coils for eliminating any remaining inhomogeneities of the magnetic field. When an image is to be generated, as in the case of a nuclear spin tomograph, an additional set of gradient coils are present in the volume under examination, for generating pulsed magnetic gradient fields. In addition, a nuclear magnetic resonance spectrometer comprises a sample head for transmitting and/or receiving r.f. pulses.
Nuclear magnetic resonance spectroscopy (NMR spectroscopy) in particular requires the use of extremely homogeneous magnetic fields. In the case of known nuclear magnetic resonance spectrometers one achieves this homogeneity with the aid of a corresponding design of the magnetic system and by electric corrective measures, in particular the use of shim coils. In investigating biological samples, especially in in-vivo spectroscopy, the following problem is, however, encountered: due to the magnetic susceptibility of the samples, an initially homogeneous magnetic field is distorted in such a way that it becomes impossible for local spectroscopy to be performed. Field distortions may be caused, for example, by the iron content in the liver of a patient who is to undergo examination in a nuclear spin tomograph.
Conventional shim systems are based on correcting expansion coefficients of the surface harmonics. They are arranged at the greatest possible diameter within the magnetic system with a view to generating the purest possible coefficients in order to avoid mutual interaction between different shim coils. Due to the large spacing from the object to be spectroscopized, it is not possible for local field distortions to be corrected.